In mammals, melanin-concentrating hormone (hMCH) is a key regulator of feeding behavior, energy homeostasis, and sleep. MCH was first identified in salmon in 1983 as a peptide (sMCH) that induced skin lightening. Despite numerous studies in different fish species, however, no clear effect on feeding was shown for sMCH. Last year, we found, in zebrafish and four other teleost fishes, two MCH genes: mch1 and mch2. Whereas mch1 perfectly resembles salmon sMCH, the mch2 gene and MCH2 peptide share genomic structure, synteny, and high homology with mammalian hMCH. Zebrafish MCH2, like mammalian hMCH, is expressed in a distinct population of hypothalamic neurons and is up-regulated upon fasting, suggesting a conservation of MCH2/hMCH regulation and function across vertebrates. However, while mammalian hypothalamus harbors thousands of MCH cells, zebrafish larval and adult brains contain more compact networks of 50 and 150 MCH neurons respectively. As MCH is not found in non-vertebrate models like Drosophila and C. elegans, the discovery of mch2 offers us a unique opportunity to explore MCH function in a simple amenable genetic model, the zebrafish. We propose in a first specific aim to characterize the MCH2 neurocircuit and to relate its activity to behavioral states. To do so, we will (i) analyze MCH2 neurons identity and fast neurotransmitter phenotype, (ii) study their arborization and connections, and (iii) precisely monitor their pattern of activity with a calcium-imaging assay in different behavioral conditions. As feeding and sleep behaviors are exclusive in their timing, it is critical to follow the firing patterns of the totality of the MCH neurons to distinguish potential feeding- related and sleep-on subpopulations. In the second specific aim, we will precisely investigate the behavioral influence(s) of MCH2. To do so we will use both a classic genetic approach and state-of-the-art optogenetic manipulation of the mch2 circuit. We will first (i) study how the MCH2 peptide and circuit regulate food intake, growth, and body weight. Further, (ii) using a novel multi-behavioral tracking system we will analyze a large spectrum of behaviors associated with energy unbalance, food search and consumption, such as exploration, anxiety, aggressiveness, bulimic like behaviors versus slow eating, and the hedonistic influence of food. Finally, (iii) we will investigate the ancestral role for MCH in sleep- wake regulation by analyzing the sleep architecture during normal sleep, induced sleep, and after sleep deprivation. In conclusion, zebrafish offers a unique situation as a transparent vertebrate to perform non-invasive observation and manipulation of small but complete neuronal networks. It will bring major insight to our understanding of how a same hypothalamic circuit times and regulates behaviors so distinct in their timing and their functions.